Infix and prefix operator.
x y is by default interpreted as CircleTimes[x, y].
Used in mathematics for various operations related to multiplication, such as direct or tensor products.
Also sometimes used to indicate a vector pointing into the page.
See The Mathematica Book: Section 1.10.8 and Section 3.10.4.
See also: \[CircleDot] , \[Times] , \[Cross] , \[Wedge] , \[CirclePlus] .